Distributed computing systems are generally well known. .See, for example, 6th International Conference on Distributed Computing systems, IEEE (May 19-23, 1986) pp. 250-259; Proceedings Real-Time Systems, IEEE (Dec. 2-4, pp. 157-163; R. Hull, et al., Virtual Resource Ring: Technique for decentralized resource management in Fault-tolerant Distributed Computer Systems. IEEE Proc. 131 Pt. E No. 2 (Mar. 1986) and U.K. Application 2069734A.
U.S. Pat. No. 4,530,051 discloses an organizational scheme for a distributed multiprocessor system in which tasks may be executed by program modules v in different processing units. It does, however, not disclose the way a unit locates the program modules concerned.
Also of interest is the "P3000 office computer system" 1 Philips Telecommunication Review, Vol. 42, No. 4, 1984, pages 214-228. This system comprises various kinds of units, such as workstations and support stations.
Workstations are provided with a keyboard, a display screen and a processing unit having one or more microprocessors. Support stations are intended to perform a number of specialized tasks for the workstations, e.g.. providing communication with other units, a printer and mass memories, for which purpose they also have their own intelligence.
The units are interconnected by means of a data network. In this way they can exchange data and make joint use of specialized equipment.
A specific office activity can be regarded as a collection of sub-tasks required to be performed. To this end, a unit has available a number of modules which perform the sub-tasks. These modules are in fact programs which are performed by the central microprocessor in the unit. Not every task or sub-task can be performed in each unit. When it is required to perform a task of this kind, the unit can search for a connection to another unit and instruct that unit to perform the required task.
The organization of this is controlled by the known office automation system itself. As far as the workstation operator is concerned, it is often not relevant to know where an order given by him is performed, provided such order is performed. To this end the units are provided with a list of all the modules present in the system with their address, which is the place in the system where the module is located.
When performance of a specific task is required, the unit searches for the associated module in its list and establishes connection therewith in accordance with the data in that list.
In this connection, problems may arise if one or more modules in the system become inoperative; for example, because the unit in which they are running malfunctions. Problems may also arise if extra modules become available, for example, as a result of an extra unit being connected to the system.
There is, therefore, a need for a mechanism which always keeps the lists of the units completely up to date, so that the system retains maximum functionality even in the event of a malfunction in one or more of the units, and, in principle can be expanded at any time by connecting extra units. As a result of such connection, however, it is possible that more than one module intended for a specific task to occur within the system. For such cases there must be a clear selection mechanism which designates one of the identical modules.
A selection mechanism for identical modules in a distributed system is disclosed in U.S. Pat. Nos. 4,466,063 and 4,430,699. The system disclosed in these patents comprises Local Systems (LS), connected to a network, each of which is provided with a Systems Interconnection Processor (SIP). An LS wanting to subcontract a task to a resource in another LS, sends a service request to its SIP. The SIP does not have a list of all resources available in the entire system, although it has one of the resources available in its own LS, and broadcasts the service request to all other LSs.
On reception of that broadcast message, the SIPs of all LSs check whether they have available the requested resource and, if so, whether the requesting LS is authorized to use it. In the affirmative, an SIP checks whether the requested resource is free to handle the service request and, if so, broadcasts an "accept" message, whereafter the requesting SIP transmits the task to the accepting SIP, which transfers the task to its LS for execution. In case the requested resource is busy, the SIP waits until it is free again and then broadcasts its "accept" message.
"Accept" broadcast messages arriving after the first one (or after the Nth one, if N resources have been asked for in the service request) are discarded and are, in fact, not transmitted from the moment a SIP that is ready to accept receives the first "accept" broadcast message from another SIP.
In order to prevent heavily loaded LSs from being selected, which would still increase their loading, the SIPs delay their "accept" messages proportionally to the loading, so that less loaded LSs will respond first and be selected.
In case of abnormal behavior of an SIP, the other SIPs, detecting errors in the messages of that SIP, ignore further messages from that SIP until they receive a normal message from that SIP again.
In this system, the selection of a resource is an entirely autonomous process formed by the interaction between the SIPs. The SIPs have no way of selecting a particular resource themselves, as a SIP does not have any information on the system (except for information on the SIPs not functioning properly) outside its own LS. Furthermore, if for economical or organizational reasons a specific task should be performed by one and the same resource in the entire system, even if there are several identical resources available (the other resources merely being reserves) the system disclosed does not provide an appropriate selection mechanism.
Accordingly, it is an object of the invention to provide an office automation system which is easy to expand and in which local malfunctions do not affect the operation of the complete system or result only in a slight degradation of performance.